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NSF, DARPA see the light on Lehigh’s wide bandgap semiconductor research

Volkmar Dierolf

Faculty members in Lehigh’s Center for Optical Technologies (COT) have won significant funding recently for wide bandgap semiconductor research that could lead to high-quality, energy-efficient lighting systems for consumers as well as lightweight bioterrorism-prevention devices for the military.

The group of physicists, electrical engineers and materials scientists has received three grants from the National Science Foundation (NSF) and one from the Defense Advanced Research Projects Agency (DARPA) of the U.S. Department of Defense.

The group’s goal is to reduce the number of defects which occur during the fabrication of compound semiconductors and which cause the loss of energy and light. Its members are conducting nanoscale experiments into the surfaces, or substrates, on which are “grown” the crystalline thin films that actually produce the light. They are also developing new techniques to refine and optimize these films, and they are exploring a new field called “spintronics,” in which light signals are employed to manipulate the spin, or orientation, of the electrons of magnetic materials.

The researchers are particularly interested in indium-gallium-nitride semiconductors, which are used to make blue light-emitting diodes (LEDs) and lasers, and in aluminum-gallium-nitride semiconductors, which are used to make ultraviolet LEDs and lasers.

Scientists and engineers hope in the near future to replace incandescent light bulbs with brighter, less power-hungry LEDs that can, when combined with solar cells, recharge from the sun and other light sources. These LEDs, which generate visible light, are already being used in traffic signals, highway signs, auto brake lights, Christmas tree lights and other applications.

Ultraviolet lasers have so far been used mostly in military applications, and are being considered for “non-line-of-sight” devices that enable soldiers to communicate without exposing their position. Ultraviolet light can also be used to detect the presence of biological or other dangerous agents.

Helen Chan

The COT’s wide bandgap semiconductor group comprises of physics professor Volkmar Dierolf; materials science and engineering professors Helen Chan, Richard Vinci and Slade Cargill; and electrical and computer engineering professors Nelson Tansu and James Hwang.

In the past two months, Chan, Vinci and Tansu have received a collaborative NSF grant, while Tansu and Dierolf have separately received single-investigator NSF grants. Chan and Vinci have also received a proof-of-concept grant from DARPA.

The NSF awards, all of which run three years, total $850,000 and come from different divisions of the federal funding agency.

“These awards are extremely competitive,” says Dierolf, who leads the wide bandgap semiconductor research program. “They are given to between 10 and 20 percent of the researchers who apply for them. For three groups or individual researchers from the COT to receive funding in one year from three different NSF divisions is very impressive.

“This success shows that the strategy of Lehigh and the COT to invest in infrastructure and in new people is paying off.”

An auspicious arrival

That research infrastructure, made possible by earlier grants from the COT and the Army Research Laboratory, includes laboratory facilities for growing as well as characterizing, or determining the material properties of, the thin crystalline semiconductor films.

Key acquisitions include metalorganic chemical vapor deposition (MOCVD) facilities, transmission electron microscopy and ultraviolet near-field microscopy.

The new equipment, says Vinci, has made it possible not only to conduct cutting-edge research into gallium-nitride semiconductors but also to pursue further funding, especially from the U.S. Department of Energy, which is requesting proposals for semiconductor research aimed at improved solid-state lighting technologies.

Vinci and Dierolf say the COT’s wide bandgap semiconductor group received another boost in 2003 when Tansu, an expert in growing optical materials and optoelectronic devices, was appointed to the faculty.

The group’s diverse composition gives it the ability to take a multifaceted approach to the problem of defects, which form as semiconductor crystals grow and cause energy and light loss. These faults, which usually occur in the crystalline structure, interfere with a material’s semiconductor properties and are a major cause of defective devices. The COT researchers are seeking to reduce the number of defects and to confine them to non-light-producing regions of the semiconductor.

Rick Vinci (left), associate professor of materials science and engineering, studies a sample with Daryl Williams of Alabama A&M University. Williams completed a summer internship at Lehigh through the department of physics.

Vinci and Chan are attempting to alter the surface of sapphire wafers, which are commonly used as substrates for semiconductor and laser materials, with nanostructures that give the surface a rough texture. This will enable Tansu to grow high-quality semiconductors using a process called nanoheteroepitaxy.

Sapphire is an expensive and extremely scratch-resistant material used in supermarket scanning machines, says Vinci. Engineers using it for a semiconductor substrate often coat it with a rough buffer layer on which the crystalline films are then grown.

“We are developing a new technique for covering the sapphire surface with carefully designed nanostructures,” he says, “so that the light-emitting materials may be grown directly on the surface with fewer defects than is currently possible. Fewer defects would result directly in brighter LEDs.”

Nanoheteroepitaxy is currently being used to grow brighter LEDs on inexpensive silicon substrates, says Tansu. “The low-cost silicon substrates,” he adds, “will significantly reduce the cost of producing LEDs, which in turn will lower the cost of solid-state lighting.”

Nelson Tansu

Tansu and his group are seeking to improve the internal quantum efficiency, or nanoscale light-producing capacity, of the semiconductor materials for LEDs and lasers.

“We are using quantum mechanics to engineer the atomic arrangement in the nanoscale active regions, such that the rate of light generation is significantly improved,” he says. “This can be done by optimizing the arrangement, composition, thickness and other parameters of the fewer than one dozen atomic layers of semiconductor films that make up a nanoscale active region.”

“A semiconductor is a system with so many parameters,” adds Dierolf, “that you have an almost unlimited number of possible tweaking scenarios. And all the relevant improvements take place at the nanoscale.”

Tansu also performs theoretical modeling of these semiconductor parameters, and is developing novel device approaches for enhancing light extraction from semiconductor LEDs.

Dierolf, the lone physicist in the group, is characterizing the optical properties of the semiconductor and laser materials at the nano-region.

A favorable spin

Dierolf is also investigating the possibility of making gallium-nitride a ferromagnetic material by doping (adding a controlled impurity to) the compound with ions from rare earth elements.

These efforts are leading him into a relatively new field known as “spintronics,” in which scientists and engineers, working at the nanoscale, use an electron’s spin rather than its full charge to create electronic devices.

“Rare earth ions introduce ferromagnetic qualities to gallium-nitride,” said Dierolf. “The electrons in ferromagnetic materials have a magnetic moment, or spin, that can point up or down like a little magnetic needle. We can use optical signals to control that spin. We’re hoping to use ferromagnetic semiconductors to make smaller devices that rely on just a few electrons for switching.”

--Kurt Pfitzer

Posted on Monday, September 24, 2007

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